In an amplifier, for example, a boost Class D amplifier, an output voltage may be modulated at five voltage levels based upon an analog input voltage level. The amplifier may include quantizers to perform pulse-density modulation (PDM) to allow the amplifier to encode the output voltage as a sequence of pulses at the five voltage levels, for example, at +VPP, +VDD, GND, −VDD, and −VPP, where VPP>VDD.
A single one-bit hysteresis quantizer-based amplifier can modulate the output at three voltage levels, for example, +VDD, GND, and −VDD, which would be insufficient for the five voltage levels in the output of a boost Class D amplifier. If multiple one-bit hysteresis quantizers are used, the modulator can encode the output voltage at the five voltage levels. However, this would degrade the output linearity and increase the complexity of comparator design of the quantizers. Additionally, the performance of the amplifier would become more sensitive to process and layout variations.
One conventional solution is to switch the range of the modulation output voltage between a normal mode of modulating between three voltage levels of +VDD, GND, and −VDD, and a boost mode of modulating between three voltage levels of +VPP, GND, and −VPP. When the input voltage is determined to be sufficiently low, the amplifier is placed in normal mode by modulating the output between three voltage levels of +VDD, GND, and −VDD. When the input voltage is determined to be sufficiently high, the amplifier is placed in boost mode by modulating the output between three voltage levels of +VPP, GND, and −VPP.
This solution may be implemented by sensing the input voltage, and switching the supply voltage between VDD and VPP, at the output power stage, depending on the mode. In other words, when in the normal mode, the booster is turn off, and the output voltage is modulated between three voltage levels of +VDD, GND, and −VDD. When in the boost mode, the booster is turned on quickly, and the output voltage is modulated between three voltage levels of +VPP, GND, and −VPP.
However, because the output voltage is modulated in a greater voltage range in the boost mode than in the normal mode, this solution imposes difficult requirements for the booster to meet, namely, to charge and discharge the required voltage quickly. The booster is subject to high voltage and current stresses during charging and discharging, and would have greater probability of stress failure. Designing the booster to withstand the stresses would increase the complexity of the amplifier.
Additionally, because the output voltage is modulated in a greater voltage range in the boost mode than in the normal mode, the modulation in the boost mode would also generate greater amount of electro-magnetic interference (EMI), which would interfere with other electronic devices or components nearby. Again, designing the electronic devices or components to withstand the EMI would increase the complexity of design.
Furthermore, when the output voltage is modulated in a greater voltage range in the boost mode, all the output power of the Class D amplifier is supplied by the booster. This would cause a high amount of power consumption.
Additionally, because the quantization error of the PDM is dependent upon the amplitude of the output pulses, when the output voltage is modulated in a greater voltage range in the boost mode than in the normal mode, the boost mode would produce higher quantization error than the normal mode.
Accordingly, there is a need for a voltage boosted Class D amplifier having increased power efficiency, less stress on the amplifier components, less EMI generation, and lower quantization error.